An engine piston must dissipate the heat energy it absorbs, from the conversion of chemical energy into heat energy and finally into mechanical work, occurring within an engine sequence.
Engine pistons are commonly made of iron or aluminum alloys. A piston has a crown with an upper surface exposed to engine combustion temperatures. The piston undercrown is exposed to crankcase fluids. A ring belt carrying compression and oil control rings extends from the edge of the crown. A piston skirt having curved sidewalls extends from the ring belt to absorb reciprocating thrust forces exerted on the piston. A pin boss may extend between the skirt walls for receiving a wrist pin for connection with a connecting rod.
In operation in an engine, the piston crown absorbs heat from an engine combustion chamber. Heat absorbed by the crown is conducted through the piston to the undercrown, the ring belt, and the skirt. Heat in the ring belt and skirt is conducted to the associated engine cylinder by direct contact and through the piston rings. Heat in the undercrown is transferred to the ring belt or dissipated to crankcase fluids, including air, oil vapors and liquid oil present in the engine crankcase and provided, in part, for piston cooling. The need for high heat transfer to control piston temperatures limits the use of higher strength piston materials, which have lower heat transfer capability.